Chemical mechanical polishing (CMP) is now widely used in the semiconductor manufacturing industry as the method of choice for planarizing any given layer prior to the deposition of the next layer. As an example, consider a dielectric layer 11, as shown schematically in FIG. 1, in which a cavity 13 has been formed and then over-filled with a layer of a metal 12, such as tungsten or copper.
To perform CMP a slurry of abrasive particles (typically SiO2 or Al2O3 particles) having a mean diameter between about 1 and 10,000 microns) suspended in an aqueous solution of hydrogen peroxide and nitric acid, is used. The hydrogen peroxide oxidizes the surface that is being polished thereby forming a brittle layer of oxide that is readily dislodged by the suspended particles.
Although the CMP process as described above works well in many respects, a few problems still remain. Two such problems are illustrated in FIG. 2. The first is exemplified by slurry particles 24, some of which tend to remain on the surface of the metal at the conclusion of CMP. These have to be removed in a separate operation and can often remain in place when not expected to. The second problem is exemplified in layer 23 which represents a final layer of oxide that remains at the conclusion of CMP. When processing continues, layer 23 may remain in place and increase the contact resistance to the next metal layer, as shown in FIG. 3 where oxide layer 23 has become interposed between the two metal plugs 12 and 31, the latter being contained within a second dielectric layer 32.
There is therefore a need for a CMP process wherein no oxide layer remains on the surface of a metal after CMP and wherein no particles of abrasive material remain either. Preferably such a process would be part of the standard CMP process as currently used so as to introduce minimum disruption into the overall manufacturing process.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,662,769, Schonauer et al. show a tungsten CMP process and post clean. In U.S. Pat. No. 6,165,956, Zhang et al. show a tungsten CMP together with NH4OH brushing in the background section. U.S. Pat. No. 5,968,280 (Ronay) shows a poly-electrolyte post CMP clean. While U.S. Pat. No. 5,868,863 (Hymer et al.) and U.S. Pat. No. 6,207,630 (Vaarstra) are related processes.